断层带通过的农庄，地表一片祥和，但断层在空照下，却也显露丑陋面貌，随时会摧毁家园。（取自美国地质调查所等网站） 地震专家8日发布爆炸性学术报告：对南加州威胁最大的圣安德里斯断层（San Andreas fault），地震周期为88年，比传统学说缩短两倍。
专家说，芮氏规模8.1地震已近在南加州人家门口！
贯穿南北加州的圣安德里斯断层，多年来被地震专家视为「加州安全的定时炸弹」，何时大地震，地震学家多年来较一致的说法，是250至450年一次，规模7.8左右。
但由尔湾加大和亚利桑纳州立大学共同研究多年、8日发表在权威学术杂志「Geology」的最新报告指出，圣安德里斯断层下次地震规模很可能高达8.1，破坏力可能北从蒙特瑞县（Monterey County）通达南加州沙顿海（Salton Sea），总长340哩。 「多年来地震学家一直同意圣安德里斯断层的活动周期为200年至250年之间，精确说法为234年」，南加大地震学家郦永刚表示，此一结论是採用航天观测地震后地表壕沟及错位深度，通过网格推算而产生。最新研究是採用雷射探测和光学扫描，发现更精确的震后痕迹，很小的错位也能发现，因此发现在圣安德里斯最近一次地震前，曾发生多次8.0左右的大地震。
据加州地震档桉记载，圣安德里斯断层对南加威胁最大的最近一次地震发生在1857年，规模7.9，震央在北加州蒙特瑞县附近泰洪堡（Fort Tejon），向南延伸200哩，在洛杉矶县北部天使国家森林一带，转而向东延伸到圣伯纳汀诺县的Cajon Pass，靠近现在的15号公路附近。 据记载，那次地震前后摇动三分钟左右，导致土壤液化，部分地区下沉严重，一些地区树木连根拔起。不过由于当时洛杉矶地居民只有4000人左右，损失有限。
之后迄今150年，圣安德里斯断层南段，即横跨洛杉矶西北100哩卡瑞索（Carrizo）平原，一直保持休眠状态。 不过通过新的雷射和光学技术最新研究显示，在1857年前，圣安德里斯断层在1417、1462、1565、1614、1713年都曾发生过规模8左右的大地震，只是那时没有人住院医治，没有留下纪录。 参与本次研究的尔湾加大地震学家Lisa GrantLudwig表示，过去大家一直喊「狼来了」，在她看来，「现在狼真的要来啦」。
南加州地震中心主任Thomas Jordan也相信，「被捆绑且超负荷的圣安德里斯巨人，现在随时都会爆发」。不过郦永刚教授认为，圣安德里斯断层近期会发生8.0以上地震的说法尚有争议。他认为，近年南加州发生8.0以上地震可能性为50%，发生7.0以上地震机率为99.5%。
郦永刚表示，由于不久前海地和墨西哥相继出现7.8以上地震，使威胁南加南部的墨西哥湾断层释放出一定能量，来自南部的地震威胁可能推迟几年；而如果按88年周期的理论，南加州也早已超过周期。 「值得注意的是，震度每差一级，震幅相差十倍，但释放的能量却相差32倍」，郦永刚指出，墨西哥湾地震大概只释放积压能量的六分之一左右，民众对大地震的防备，仍不可掉以轻心。
（世界日报 2010-10-10）
San Andreas fault capable of magnitude 8.1 earthquake over 340-mile swath of California, researchers say [Updated]
The "Big One" on the San Andreas fault just got a little bigger.
New research showing a section of the fault is long overdue for a major earthquake has some scientists saying that the fault is capable of a magnitude 8.1 earthquake that could run 340 miles from Monterey County to the Salton Sea.
Whether such a quake would happen in our lifetime had been a subject of hot debate among scientists. That's because experts had believed that a major section of the southern San Andreas, which runs through the Carrizo Plain 100 miles northwest of Los Angeles, would remain dormant for at least another century.
But that rosy hypothesis seemed to be shattered by a recent report in the journal Geology, which said that even that section of the San Andreas is far overdue for the "Big One." [Updated, Oct. 9: The report, published in August, was written by Sinan Akciz and Lisa Grant Ludwig of UC Irvine, and J. Ramon Arrowsmith and Olaf Zielke of Arizona State University.]
Now, according to U.S. Geological Survey seismologist Lucy Jones, it is entirely possible that all 340 miles of the southern San Andreas could be ready to erupt at any time. Such a scenario would trigger a magnitude 8.1 earthquake, said Thomas Jordan, director of the Southern California Earthquake Center, a calculation with which Jones agreed.
"All of it has plenty enough stress for it to be ready to go," Jones said. "The biggest implication of [the report] is that it increases the likelihood that when we do have a big earthquake, it will grow into the 'wall-to-wall' rupture."
[Updated, Oct. 9: Such a temblor could cause much more damage because with a longer stretch of the fault rupturing, a larger area is exposed to the quake, and the shaking would last longer.]
The walls Jones is referring to are the boundaries of the southern San Andreas, which begins in the Salton Sea and ends in the town of Parkfield in Monterey County. Scientists consider the southern San Andreas fault as one segment generally because it behaves the same -- it rarely rumbles, but when awakened, the shaking can be devastating.
In contrast, the section of the San Andreas north of Parkfield up to Hollister in San Benito County behaves differently. That section constantly moves at a creep -- meaning stress is relieved regularly, so large quakes don't occur there.
Large quakes haven't occurred anywhere on the southern San Andreas for more than a century, making it a sleeping giant that has been building stress for so long it could snap at any moment.
"My concern is that we will get a series of large earthquakes along the San Andreas fault," Jordan said. The last "Big One" to rip through Southern California occurred in 1857, when an estimated magnitude-7.9-quake, ruptured 200 miles of fault between Monterey and San Bernardino counties. It wasn't a wall-to-wall quake: It stopped near the Cajon Pass, near the present-day 15 Freeway, probably because the fault south of it shook just a few decades earlier, in 1812, Jones said. Because the 1812 quake had relieved tectonic tension in that area, it effectively put a brake to the 1857 quake from moving further south.
But with the San Bernardino County section of the fault now having accumulated two centuries' worth of strain, there may not be any brakes now. "Can I imagine the 1857 earthquake happening again and stopping at the Cajon Pass? Probably not," Jones said. "Once you have a big slip, you're more likely to move along down the fault," Jones said. "If the rupture has been made ... that’s a lot of momentum that will keep the rupture moving down the fault."
The San Andreas has long been considered one of the most dangerous faults in Southern California because of its length. Not only do longer faults produce bigger quakes, they emit a type of shaking energy that can travel longer distances.
"So a much larger area is affected by a really large earthquake," Jones said.
In 2008, seismologists developed a scenario for a large earthquake on the San Andreas -- a magnitude 7.8 shaker that begins at the Salton Sea and barrels northwest along the fault toward San Bernardino and Los Angeles counties.
-- Rong-Gong Lin II
Photo: Quake researchers study a portion of the San Andreas fault. Ricardo DeAratanha / Los Angeles Times; Map: U.S. Geological Survey
